As is well known in the lithographic art, a support material such as a protectively coated metal plate is coated with a sensitizer or photosensitive layer containing a diazo compound. When the resulting photosensitized plate is exposed to a suitable light source through a transparency or a means for selective transmission of such light, image and nonimage areas are formed. Thereafter the plate is developed to provide oleophilic ink-receiving areas and hydrophilic water-receiving areas, so that it can be used in a lithographic press.
One disadvantage of the commercially available lithographic printing plates is that they do not have the relatively high exposure sensitivity required to either effectively or economically produce an image by projection exposure on such printing plates. Moreover, any image so produced will be lost after several hundred or less impressions.
One object of the present invention is to overcome the disadvantages discussed above, thereby permitting effective use of projection exposure to image lithographic printing plates having an overlying layer, containing a diazo compound without deleteriously affecting the other desirable characteristics of the printing plate.